Introduction to Writing Electron Configurations

Key Concepts

• Higher number in the periodic table: Mass number
• Lower number in the periodic table: Atomic number (integer)
• First energy level: 1s sublevel
• Second energy level: 2s and 2p sublevels
• Third energy level: 3s, 3p, and 3d sublevels
• Electron capacity per sublevel:
  • s: 2 electrons
  • p: 6 electrons
  • d: 10 electrons
  • f: 14 electrons

Writing Electron Configuration for Atoms

Nitrogen

• Atomic number: 7 (7 electrons)
• Configuration steps:
  1. 1s²: 2 electrons
  2. 2s²: 2 electrons
  3. 2p³: 3 electrons
• Final configuration: 1s² 2s² 2p³

Aluminum

• Atomic number: 13 (13 electrons)
• Configuration steps:
  1. 1s²: 2 electrons
  2. 2s²: 2 electrons
  3. 2p⁶: 6 electrons
  4. 3s²: 2 electrons
  5. 3p¹: 1 electron
• Final configuration: 1s² 2s² 2p⁶ 3s² 3p¹

Writing Electron Configuration for Ions

Fe²⁺ Ion (Iron)

• Atomic number: 26 (26 electrons in neutral atom)
• Fe²⁺ has 24 electrons (lost 2 electrons)
• Configuration steps (for neutral Fe):
  1. 1s²: 2 electrons
  2. 2s²: 2 electrons
  3. 2p⁶: 6 electrons
  4. 3s²: 2 electrons
  5. 3p⁶: 6 electrons
  6. 4s²: 2 electrons
  7. 3d⁶: 6 electrons
• Final configuration for Fe²⁺ (remove 2 electrons from 4s): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶

Cl⁻ Ion (Chlorine)

• Atomic number: 17 (17 electrons in neutral atom)
• Cl⁻ has 18 electrons (gained 1 electron)
• Configuration steps (for Cl⁻):
  1. 1s²: 2 electrons
  2. 2s²: 2 electrons
  3. 2p⁶: 6 electrons
  4. 3s²: 2 electrons
  5. 3p⁶: 6 electrons
• Final configuration for Cl⁻: 1s² 2s² 2p⁶ 3s² 3p⁶

General Guidelines

• For transition metals, write the electron configuration of the parent atom first, then adjust for ion charge.
• For non-transition elements, directly calculate the total number of electrons in ions and write their configuration accordingly.
• Add electrons for negatively charged ions.
• Subtract electrons for positively charged ions.